Add configuration file parameter "config_directory"

This enables explicit provision of an external configuration file
directory, which if set will be passed to "pg_ctl" as the -D
parameter. Otherwise "pg_ctl" will default to using the data directory,
which will cause some operations to fail if the configuration files
are not present there.

Note this is implemented primarily for feature completeness and for
development/testing purposes. Users who have installed "repmgr" from
a package should not rely on "pg_ctl" to stop/start/restart PostgreSQL,
instead they should set the appropriate "service_..._command" for their
operating system. For more details see:

    https://repmgr.org/docs/4.0/configuration-service-commands.html

Note: in a future release, the presence of "config_directory" in repmgr.conf
will be used to implictly set "--copy-external-config-files=samepath" when
cloning a standby; this is a behaviour change so will be implemented in the
next major realease (repmgr 4.1).

Implements GitHub #424.

configfile.h

@@ -73,6 +73,7 @@ typedef struct
 	char		conninfo[MAXLEN];
 	char		replication_user[NAMEDATALEN];
 	char		data_directory[MAXPGPATH];
+	char		config_directory[MAXPGPATH];
 	char		pg_bindir[MAXPGPATH];
 	int			replication_type;
 
@@ -159,7 +160,7 @@ typedef struct
 
 #define T_CONFIGURATION_OPTIONS_INITIALIZER { \
 		/* node information */ \
-		UNKNOWN_NODE_ID, "", "", "", "", "", REPLICATION_TYPE_PHYSICAL,	\
+		UNKNOWN_NODE_ID, "", "", "", "", "", "", REPLICATION_TYPE_PHYSICAL,	\
 		/* log settings */ \
 		"", "", "", DEFAULT_LOG_STATUS_INTERVAL,	\
 		/* standby clone settings */ \